1. Field of the Invention
The invention relates to aircraft gas turbine engines and, particularly, for such engines having multiple fan bypasses.
2. Description of Related Art
A conventional gas turbine engine includes a core engine having in serial, axial flow relationship, a high pressure compressor to compress the airflow entering the core engine, a combustor in which a mixture of fuel and the compressed air is burned to generate a propulsive gas flow, and a high pressure turbine which is rotated by the propulsive gas flow and which is connected by a radially inner relatively large diameter shaft to drive the high pressure compressor. A typical bypass turbofan engine adds a low pressure turbine aft of the high pressure turbine and adds a forward fan forward of the high pressure compressor.
A typical variable bypass ratio design as, disclosed in U.S. Pat. No. 4,068,471, includes one or more forward rows of fan rotor blades in the front fan. The fan rotor blades are connected to a radially inner relatively small diameter drive shaft, which runs through the hollow large diameter shaft and is driven by the low pressure turbine. An aft fan includes one or more aft rows of fan rotor blades connected to the larger-diameter drive shaft and is driven by the high pressure turbine. The aft fan is disposed in serial, axial flow relationship between the forward fan and the high pressure compressor. A variable area bypass injector is located between the forward and aft fans to vary the amount of air entering a first inlet of a fan bypass duct which varies the fan bypass ratio of the engine (i.e., the ratio of the air flowing through the fan bypass duct to the air flowing through the core engine) from which comes the term variable cycle to describe the engine. The fan bypass duct has a second inlet located aft of the aft row of fan blades. Control of airflow directed into the first and second bypass duct inlets was typically accomplished by selector valve mechanisms and some more particular valves called variable bypass injectors commonly referred to as VABIs.
An engine having a fan bypass duct with two or more inlets may be called a multiple bypass stream variable cycle gas turbine engine. A row of stator vanes is typically located just forward of each forward and aft row of fan blades. Selected rows of stator vanes are variable, typically variable angle, to vary the angle of the flow seen by the rotor blades. Some of the engine thrust comes from the propulsive gases exiting the core engine and some from the airflow exiting the fan bypass duct. Variable cycle engines with core driven fan stages have been designed to effectively increase fan bypass stream pressure ratio by using the tip section on the first stage of the core to boost the bypass stream pressure ratio. One example of such a design is disclosed in U.S. Pat. No. 5,809,772 entitled “Turbofan Engine With A Core Driven Supercharged Bypass Duct”. They typically utilize a series of guide vanes and doors to modulate the flow to the bypass stream.
There is a need for an aircraft gas turbine engine to operate at high fan hub and bypass stream pressure ratios to provide high specific thrust at takeoff and climb power settings and to operate at low bypass stream pressure ratios to provide good specific fuel consumption during reduced power cruise operation. Although a conventional fan can be made to operate at reduced pressure ratios (limited by choking of the flow within the blading), this will also reduce the hub pressure ratio that supercharges the core, thereby, reducing cycle efficiency and negating the improvements in specific fuel consumption. Typically, there is also a large reduction in fan efficiency at low fan pressure ratios which also reduces improvements in specific fuel consumption.
It is highly desirable to have an aircraft gas turbine engine that can modulate bypass flow from a fan section around a core engine to the bypass stream and to effectively operate at high fan hub and bypass stream pressure ratios to provide high specific thrust at takeoff and climb power settings and to operate at low bypass stream pressure ratios to provide good specific fuel consumption during reduced power cruise operation.